As part its continuing restructuring of top management, USF&G; Corp. has announced the appointment of two top executives to head its investment and computer operations.

J. Michael Gaffney was appointed executive vice president and chief investment officer for the giant Baltimore-based insurance company.

He will be responsible for the management of the USF&G;'s investment portfolio and the company's Investment Management Group.

Gaffney was the chairman and chief executive officer of Security Pacific Investment Managers, a subsidiary of Security Pacific Corp, a Los Angeles-based bank holding company.

He was also the chairman of Bunker Hill Income Securities, a closed-end bond fund, and had responsibility for all institutional subsidiaries of the Security Pacific Investment Group.

In these different positions, Gaffney oversaw the investment of $12.7 billion, generating $15.5 million in revenues.

From 1981 to 1987 he was the manager of the Fixed Income Portfolio for T. Rowe Price Associates Inc., a Baltimore mutual funds company.

John Roblin was elected to the position of senior vice president with the responsibility for the company's management information systems area. He had previously been a senior vice president, managing director of information systems of Chubb & Son, Inc., an insurance company in New Jersey, where he was responsible for data processing operations, strategy and management.

USF&G;, which has assets of $13.9 billion, includes subsidiaries that provide property and casualty insurance, life insurance and investment services.

The company's principal subsidiary is the United States Fidelity and Guaranty Co.